Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since the mid 1970s. Because of low dew point and virtual absence of carbon dioxide, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been used extensively for bright annealing non-ferrous metals and alloys such as copper and gold.
A major portion of nitrogen used by the heat treating industry has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally pure and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen is indeed inexpensive, but it contains 0.2 to 5%, residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in heat treating furnaces very difficult, if not impossible.
Attempts have been made to use reducing gases such as a hydrocarbon and hydrogen along with non-cryogenically produced nitrogen to produce atmospheres suitable for heat treating or bright annealing parts in furnaces but with limited success even with the use of an excess amount of a reducing gas. The problem has generally been related to surface oxidation of the heat treated or annealed parts in the furnace.
A mixture of non-cryogenically produced nitrogen and hydrogen has been used for annealing copper and described in papers titled, "The Use of Non-Cryogenically Produce Nitrogen in Furnace Atmospheres", published in Heat Treatment of Metals, pages 63-67, March 1989 and "A Cost Effective Nitrogen-Based Atmosphere for Copper Annealing", published in Heat Treatment of Metals, pages 93-97, April 1990. These papers describe that a heat treated copper product was slightly discolored when all the gaseous feed containing a mixture of hydrogen and non-cryogenically produced nitrogen with residual oxygen was introduced into the heating zone of a continuous furnace. It is, therefore, clearly evident that according to the prior art, copper cannot be bright annealed with a mixture of non-cryogenically produced nitrogen and hydrogen in continuous furnaces.
U.S. Pat. No. 5,057,164 discloses and claims a method for producing an atmosphere suitable for heat treating metals from non-cryogenically produced nitrogen in continuous furnaces by reacting residual oxygen with hydrogen or carbon monoxide in the heating zone followed by extracting a part of the atmosphere from the heating zone and introducing it into the cooling zone of the furnace. Unfortunately, this process requires a large amount of hydrogen or carbon monoxide to provide a high pH.sub.2 /pH.sub.2 O or pCO/pCO.sub.2 ratio (or reducing environment) in the furnace, making it uneconomical for bright annealing, brazing, and sintering non-ferrous metals and alloys.
Researchers have explored numerous alternative ways of using noncryogenically produced nitrogen for heat treating metals in continuous furnaces. For example, furnace atmospheres suitable for bright annealing copper, brazing copper, and sintering copper and copper alloys have reportedly been generated from non-cryogenically produced nitrogen by converting residual oxygen to moisture with hydrogen gas in external units prior to feeding atmospheres into the furnaces. Such atmosphere generation methods have been disclosed in detail in U.S. Pat. No. 3,535,074, Australian Patent Applications AU45561/89 and AU45562/89 dated Nov. 24, 1988, and European Patent Application 90306645.4 dated Jun. 19, 1990. Unfortunately, these processes are not cost-effective because they require expensive hydrogen to maintain a reducing environment in the furnace.
U.S. Pat. No. 4,931,070 and French Patent Publications 2,639,249 and 2,639,251 dated Nov. 24, 1988 disclose and claim processes for producing atmospheres suitable for heat treating metals from non-cryogenically produced nitrogen by converting residual oxygen to moisture with hydrogen in external catalytic units followed by extraction of moisture prior to introducing the atmosphere into a furnace. These methods are not cost effective because they 1) require expensive hydrogen to maintain a reducing environment in the furnace and 2) there are significant costs associated with extracting moisture from the atmosphere.
U.S. Pat. No. 5,069,728 discloses and claims a process for producing atmospheres suitable for heat treating from non-cryogenically produced nitrogen by simultaneously introducing 1) non-cryogenically produced nitrogen along with hydrogen and carbon monoxide in the heating zone and 2) non-cryogenically produced nitrogen pretreated to convert the residual oxygen to moisture with hydrogen in an external catalytic reactor or nitrogen gas free of oxygen in the cooling zone of a continuous furnace. Unfortunately, this method requires expensive hydrogen or carbon monoxide to maintain reducing environment in the furnace, making it uneconomical for bright annealing, brazing, and sintering non-ferrous metals and alloys.
Based upon the above discussion, it is clear that there is a need for processes for generating low-cost atmospheres for bright annealing, brazing, and sintering non-ferrous metals and alloys from non-cryogenically produced nitrogen. Additionally, there is a need to develop processes which are cost effective and eliminate the need of expensive hydrogen gas.